1. Field of the Invention
The present invention relates to a communication system, especially to a method for transmitting SRS in an LTE communication system and an apparatus using the same.
2. Description of the Related Art
3GPP (The 3rd Generation Partner Project) standardization organization is working out a next generation of wireless communication standard which is named LTE (Long Term Evolution). In a physical layer interface, the new standard adopts OFDM (Orthogonal Frequency Division Multiplexing) technology, which is different from conventional CDMA (Code Division Multiple Access) technology. OFDMA is used in downlink and SCFDMA (Single Carrier Frequency Division Multiple Access) is used in uplink. The technology used in the new standard is effective to resist multi-path propagation, with the adoption of frequency domain equalization reducing complexity of the conventional time domain equalization, and is more suitable to bandwidth high-speed data transmission.
From a point of view of air-interface, the LTE standard techniques can be divided into two categories: a TDD (Time Division Duplex) system and an FDD (Frequency Division Duplex) system. The LTE system supports variable bandwidths. And typical bandwidths include 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz, which can meet demands of different scenarios.
FIG. 1 illustrates a physical layer frame structure for a LTE FDD system in which a length of radio frame (101) is 10 ms, consisting of ten equally sized radio sub-frames (102) of 1 ms length. Each radio sub-frame consists of two equally sized timeslots (103) of 0.5 ms length.
FIG. 2 illustrates a PHY layer frame structure for LTE TDD system. As shown in FIG. 2, a length of radio frame (201) is 10 ms, consisting of ten equally sized radio sub-frames (204) of length 1 ms. Each five continuous radio sub-frames consists a half-frame (202) of length 5 ms. Different from the LTE-FDD system, a second (211) and seventh (212) radio sub-frame in LTE-TDD radio frame are two special sub-frames. A length of the special sub-frame is 1 ms, consisting of three special slots, indicating DwPTS (205 or 208), GP (206 or 209) and UpPTS (207 or 210) respectively. The lengths of the three special slots are variable and are defined by system, and the total length is 1 ms. The length of UpPTS can be 0, 1 or 2 SCFDMA symbols. If the length of UpPTS is 2, UpPTS is used to transmit the uplink Short RACH or Uplink SRS signal or both the Short RACH and SRS signal. If the length of UpPTS is 1, UpPTS is used to transmit the uplink SRS signal. The other eight sub-frames except the special two are respectively consist of two slots (203) of length 0.5 ms.
In the LTE system, according to network scheduling, UE (User Equipment) sends an SRS (Sounding Reference Signal) to eNodeB (evolved NodeB). The SRS signal is used to: according to an analysis result of the SRS signal, eNodeB estimates a quality of channel which is used for transmitting SRS from UE to eNodeB and scheduling data according to frequency selective characteristics; eNodeB performs timing tracking for UE by analyzing the SRS signal and performs a close-loop power control. According to a current standardizing process, main conclusions for SRS transmission in LTE FDD system include: eNodeB broadcasts the SRS in a designated cell as needed and SRS is transmitted in some sub-frame in a designated cell periodically. A period is selected from {2, 5, 10, 20, 40, 80, 160, 320}ms. After the UE receives the SRS in the designated cell, the ODFM symbol resource occupied by the SRS is not used when transmitting uplink data. In order to perform the transmission of SRS, UE should receive a user-designated SRS signal transmitted from the network. The signal informs the user of the OFDM symbol resource used to transmit SRS. Currently, there is no description for transmitting the SRS of designated UE in a PHY layer specification which is accomplished in LTE.
Nowadays, a basic idea in the standard for the user-designated SRS signaling is that the signaling includes three parts: Duration, Period and Offset in which, the duration can use 1 bit to indicate that just one snapshot or infinite. The period value is selected from {2, 5, 10, 20, 40, 80, 160, 320}ms. In LTE FDD, the offset is a time between twice transmission time of each OFDM symbol of SRS from the beginning of the SRS period, and the basic unit is 1 ms. In LTE TDD, definition of offset is different from that in LTE FDD. Since in LTE TDD, SRS can be transmitted in UpPTS or the other uplink sub-frame, the uplink sub-frame may be discontinuous and UpPTS occupy two OFDM symbols at most, the offset is defined as an interval between an OFDM symbol position used to transmit SRS and an OFDM symbol position used to transmit SRS until the period of SRS transmission starts. For example, if the SRS symbol position at the period beginning is defined as 0, a symbol position used to transmit SRS is 3 means that the interval between the two symbols is 3, there is at most 2 OFDM symbol positions can be used to transmit SRS.
The manner of transmitting SRS in LTE TDD is mainly the same as that in LTE FDD. However the system structure of LTE TDD is different from of that in LTE FDD. Difference is that in LTE TDD, a half-frame of length 5 ms has both uplink sub-frame and downlink sub-frame, a number of uplink sub-frames and downlink sub-frames is configured by the network. In some configuration, a half-frame of length 5 ms at least has one uplink sub-frame (exclude UpPTS). According to a principle that only one SRS is transmitted in one uplink sub-frame, there is only one SRS transmission in every 5 ms, and the system can't achieve the SRS transmission with a 2 ms period. Therefore, the performance of SRS transmission by UE is deteriorated in a fast Time-varying channel.
Based on the difference between the LTE TDD and LTE FDD, the current configuration of the 2 ms transmission period for SRS in the LTE FDD cannot be used in the LTE TDD system.